Author: Ian Dunn Publisher: ISBN: Category : Languages : en Pages : 23
Book Description
The enhanced growth of ignition kernels through modulation of nanosecond pulsed high-frequency discharges is investigated quantitatively in a reactive flow. High-frequency discharge and new notions of rotational temperature coupling per subsequent pulse (
Author: Nicholas E. Deak Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
The application of non-equilibrium plasma (NEP) pulses to ignite hydrocarbon/air mixtures has emerged as a promising technology for ensuring reliable ignition and combustion stability in difficult regimes. Despite its promise, major challenges and limitations still remain, particularly in the realm of conducting high-fidelity multidimensional numerical studies. The aim of this thesis is to develop, implement, and apply a robust and efficient computational framework that addresses some of these shortcomings. As a preliminary step, the ignition of hydrocarbon/air mixtures by nanosecond pulsed discharges (NSPD) is investigated using a zero dimensional isochoric adiabatic reactor. A state-of-the-art two-temperature kinetics model, comprised of an experimentally-verified NEP plasma mechanism coupled with a hydrocarbon/air oxidation mechanism, is used. Simulations are performed to assess the impact of changing initial pressure (which varies from 1 to 30 atm) and fuel type (methane and ethylene). It is found that at lower pressures, plasma-assisted ignition (PAI) imparts a benefit over thermal ignition for both fuel types, through the creation of combustion radicals O, H, and OH. At higher pressures, PAI of methane loses efficiency compared to ethylene, due to a lack of available H radicals (which are swept up by O2), which limits the conversion of formaldehyde to formyl. Next, a robust and efficient framework for simulating NSPD in multiple dimensions is developed. The reactive Navier-Stokes equations are extended to include a drift-diffusion plasma-fluid model with a local field approximation (LFA) in a finite-volume solver, which uses an adaptive mesh refinement (AMR) strategy to address the wide separation of length scales in the problem. A two-way coupling strategy is used whereby the plasma-fluid model and reactive Navier-Stokes equations are integrated simultaneously. An effective grid refinement approach is developed in order to ensure that the physical structures that arise during and after the NSD (including the propagating streamer heads, electrode sheaths, and expansion wave during the inter-pulse period) are resolved efficiently. Severe time step size restrictions that arise from the explicit temporal integration of the transport terms are mitigated through use of a semi-implicit approach for solving Poisson's equation for the electric potential, and an implicit strategy for evaluating electron diffusion terms. A series of numerical studies are then conducted to investigate the ignition and propagation phases of atmospheric air streamers in axisymmetric discharge configurations. A range of conditions and configurations are explored to characterize the streamer, with an emphasis on the cathode sheath region, which supports steep gradients in charged species number densities as well as strong electric fields. The formation of the cathode sheath is shown to be a consequence of processes at the cathode surface, driven by electron losses at the boundary, and a strong dependence on the emission of secondary electrons. Finally, the oxidation of ethylene/air mixtures mediated by NSPD is simulated in a pin-to-pin configuration. All phases of the plasma discharge are simulated explicitly (including streamer ignition, propagation, and connection, as well as the subsequent spark phase), along with the evolution of the plasma during the inter-pulse period. Temporally and spatially-resolved results are presented, with an emphasis on the analysis of heating and energy deposition, as well as of the evolution of the concentration of active particles generated during the NSPD and their influence on ignition. The impact of pin thickness is discussed, and it is shown that the use of thin pins limits the regions of energy deposition and temperature increase near the pin tips, hindering ignition. The application of multiple pulses is explored and it is shown that multiple voltage pulses of the same strength leads to substantial energy deposition and temperature increases O(1,000 - 10,000 K) near the pin tips. Discussion is rounded out by addressing how pulse frequency and initial mixture control the generation of active particles and combustion products. Finally, recommendations for future work are provided
Author: Elliot Hing-Gwok Lim Publisher: ISBN: Category : Languages : en Pages : 82
Book Description
Lewis and von Elbe's data on minimun spark ignition energies in gases have been the standard for many years, howewer, these data still do not agree with the most detailed computational models available. Current models predict lower minimun ignition energy values. An experimental program was initiated to better characterize the ignition source and to study its effect on minimun ignition energies. The apparatus constits of a laser ignition source operating either as a Q-switched nanosecond or a pulsed mode-locked picosecond laser. For mixtures of varying equivalence ratio, the minimun ignition energy was bracketed through repeated trials at varying laser spark energies. The current study has shown that laser sparks of different pulse durations produce larger minimun ignition energies (MIEs) in comparison to electric discharge and that the MIE is a function of laser pulse duration. The laser ignition experiments are consistent with Lewis and von Elbe's measurements for lean and rich mixtures ; howewer, neared to stoichiometric mixtures, the laser ignition values are significantly higher. To better understand these differences, measurements of the spark size and dynamics of spark propagation were made using visible emission, schlieren and interferometric imaging. It is found that although the laser energy is deposited in a small volume (typically a nearly-spherical region of 0.3 mm radius), the thermal energy deposited by the spark expands to a size much larger than the calculated minimun flame kernel on a time scale much shorter than the time scale for ignition. This indicates that the energy remaining within the initial deposition region is only a small fraction of the total deposited energy. Consequently, the energy deposition process does not necessarily couple efficiently into the ignition process. The picosecond and nanosecond pulse duration sparks were also seen to exhibit different expansion properties, with the nanosecond source yielding (for the same energy deposition) larger kernels after completion of the expansion process. From these observations it is concluded that the minimun ignition energies are strongly influenced by the dynamics of spark propagation, which in turn vary for different energy sources. In particular, it appears unlikely that the short-duration laser ignition sources are capable of yielding minimun ignition energies as low as longer-duration electric spark sources.
Author: Michael Günther Publisher: Springer ISBN: 3319455044 Category : Technology & Engineering Languages : en Pages : 324
Book Description
The volume includes selected and reviewed papers from the 3rd Conference on Ignition Systems for Gasoline Engines in Berlin in November 2016. Experts from industry and universities discuss in their papers the challenges to ignition systems in providing reliable, precise ignition in the light of a wide spread in mixture quality, high exhaust gas recirculation rates and high cylinder pressures. Classic spark plug ignition as well as alternative ignition systems are assessed, the ignition system being one of the key technologies to further optimizing the gasoline engine.
Author: M. Capitelli Publisher: Springer Science & Business Media ISBN: 9400902670 Category : Science Languages : en Pages : 798
Book Description
Molecular Physics and Hypersonic Flows bridges the gap between the fluid dynamics and molecular physics communities, emphasizing the role played by elementary processes in hypersonic flows. In particular, the work is primarily dedicated to filling the gap between microscopic and macroscopic treatments of the source terms to be inserted in the fluid dynamics codes. The first part of the book describes the molecular dynamics of elementary processes both in the gas phase and in the interaction with surfaces by using quantum mechanical and phenomenological approaches. A second group of contributions describes thermodynamics and transport properties of air components, with special attention to the transport of internal energy. A series of papers is devoted to the experimental and theoretical study of the flow of partially ionized gases. Subsequent contributions treat modern computational techniques for 3-D hypersonic flow. Non-equilibrium vibrational kinetics are then described, together with the coupling of vibration-dissociation processes as they affect hypersonic flows. Special emphasis is given to the interfacing of non-equilibrium models with computational fluid dynamics methods. Finally, the last part of the book deals with the application of direct Monte Carlo methods in describing rarefied flows.
Author: Henning Bockhorn Publisher: Springer Science & Business Media ISBN: 3642851673 Category : Science Languages : en Pages : 595
Book Description
Soot Formation in Combustion represents an up-to-date overview. The contributions trace back to the 1991 Heidelberg symposium entitled "Mechanism and Models of Soot Formation" and have all been reedited by Prof. Bockhorn in close contact with the original authors. The book gives an easy introduction to the field for newcomers, and provides detailed treatments for the specialists. The following list of contents illustrates the topics under review:
Author: Timothy J. Barth Publisher: Springer Science & Business Media ISBN: 366203882X Category : Mathematics Languages : en Pages : 594
Book Description
The development of high-order accurate numerical discretization techniques for irregular domains and meshes is often cited as one of the remaining chal lenges facing the field of computational fluid dynamics. In structural me chanics, the advantages of high-order finite element approximation are widely recognized. This is especially true when high-order element approximation is combined with element refinement (h-p refinement). In computational fluid dynamics, high-order discretization methods are infrequently used in the com putation of compressible fluid flow. The hyperbolic nature of the governing equations and the presence of solution discontinuities makes high-order ac curacy difficult to achieve. Consequently, second-order accurate methods are still predominately used in industrial applications even though evidence sug gests that high-order methods may offer a way to significantly improve the resolution and accuracy for these calculations. To address this important topic, a special course was jointly organized by the Applied Vehicle Technology Panel of NATO's Research and Technology Organization (RTO), the von Karman Institute for Fluid Dynamics, and the Numerical Aerospace Simulation Division at the NASA Ames Research Cen ter. The NATO RTO sponsored course entitled "Higher Order Discretization Methods in Computational Fluid Dynamics" was held September 14-18,1998 at the von Karman Institute for Fluid Dynamics in Belgium and September 21-25,1998 at the NASA Ames Research Center in the United States.
Author: Holger Babinsky Publisher: Cambridge University Press ISBN: 1139498649 Category : Technology & Engineering Languages : en Pages : 481
Book Description
Shock wave-boundary-layer interaction (SBLI) is a fundamental phenomenon in gas dynamics that is observed in many practical situations, ranging from transonic aircraft wings to hypersonic vehicles and engines. SBLIs have the potential to pose serious problems in a flowfield; hence they often prove to be a critical - or even design limiting - issue for many aerospace applications. This is the first book devoted solely to a comprehensive, state-of-the-art explanation of this phenomenon. It includes a description of the basic fluid mechanics of SBLIs plus contributions from leading international experts who share their insight into their physics and the impact they have in practical flow situations. This book is for practitioners and graduate students in aerodynamics who wish to familiarize themselves with all aspects of SBLI flows. It is a valuable resource for specialists because it compiles experimental, computational and theoretical knowledge in one place.
Author: Ian Dunn
Author: Nicholas E. Deak
Author: Elliot Hing-Gwok Lim
Author: Michael Günther
Author: M. Capitelli
Author: Henning Bockhorn
Author: Maximilian Lackner
Author: Timothy J. Barth
Author: J. T. Anthony
Author: Holger Babinsky